A server I helped to setup recently has its OS re-installed, and I
know that because I can no longer log onto it. I used to have an
ssh-tunneling account set up on that server too for people who need to
reach outside a certain great firewall, and it’s also gone with the
revamp–my father being one of the users. So I decided to add an
ssh-tunneling account to my router (with dd-wrt)

What I need is the public key of whoever is going to use it, in this
case my father’s–alternatively I can also make a pair of
private/public keys and distribute the private key to the target
users, but that kind of defies the purpose of a “private” key.

All I need to do is to put the following snippet into my router’s
startup script, which can be modified from the web interface of
dd-wrt: administration -> commands.